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MG MGB GT V8 Factory Originals Technical - IRS

Has any one tried adapting some existing IRS?.There are so many around nowdays something must lend itself to adaption. I saw a BMW that seemed close. The cost of various 'ready made" such as NG makes them prohibitive.

I will be interested to see your findings here Pete, as this is an avenue I will be exploring when I start my V8 conversion. I did have a brief chat with a company in Sydney called Difftrans (i think) they seemed to be aware of a few possible options although I haven't looked into it any further. One they did mention for adaptability and strength (although not sure if this extended to V8 power as I was researching for upgrade to std engined B at the time) was the KIA sportage *winces a little* Said they had used it in a few race modified vehicles and found it to be quite strong - may be worth a look?

I thought that the KIA Sportage uses a strut type shock. Can a strut type suspension be converted to coilovers without much re-engineering? If so, that would open the choices a great deal.
Bob Fish


I assume you've seen Evan Amaya's webpages using Jag IRS? I have seen another web page in the UK also.

If you are considering BMW then you could look at the old Datsun (wincing also) 240k, 1600 (P510) and 180B. I am led to believe the early Datsun rear ends were built under license to BMW.

For that matter the VR & VS Commodore had IRS in the "Acclaim" versions (the family car is a VR).


Dom you are right - some of the Commodores have IRS - but i think it would be a difficult/complex task to shorten it down to suit an MG, due to the shape of the parts involved. One of the major plusses for the Jag rear (as in Evans conversion)is that its design means simply shortening straight shafts to get the necessary width.
Yes there are numerous IRS setups but ideally we would look for one that is simple in design to allow for easier modification. Any progress Peter?

Steve, not much, though not much hurry either. Been looking obsesivly at various rear supensions and seem to see IRS everywhere. Saw a Merc' 4WD that had a wide square diff frame that would lend itself to shortening. It seems to me even at this early stage that IRS would be similiar in cost to shortening a limited slip live axle. You would need a square frame for the diff' to bolt into(that the suspension could also bolt to) . Then a couple of links from the front corners of the frame to the front spring hangers. Not hard to make up at all. The only expensive bit would be shortening the drive shafts, which might not even be necessary if you can find an offset variety of IRS.I've noticed a few offset IRS set ups on various 4WD's, which gives pause for thought ie use two LHS's

I am glad this thread has opened up as I have been interested in this application for a long time and wondered if one from a Jappo would fit with modification. Whoever pioneers this conversion, and shares it, will earn the gratitude of a lot of people!
maurie prior

There are a lot of small sized SUV's out there with FWD that have a rear suspension that looks ready made for IRS, only they don't have a diff or driveshafts installed. Food for thought. My ford T-Bird IRS has made exactly no progress but maybe over the winter I'll get something done on it. I have a mill to put back together first.

Jim Blackwood

Now 4wds are likly to have stronger than usual diff's and drive shafts. Limited slip as well.
Wonder how interchangeable the drive shafts (front for rear) are on 4WD's. I would imagine that the factory would tend to keep tooling for splines etc the same front and back.I'm thinking about my old subaru (long ago sold/towed away). The drive shafts on the front were shorter than the rear because of the wider gearbox. 4 bolt wheel pattern too. The rear trailing arms set up on the Subaru are of course not suitable but there are other 4WDs around. Saw a double wish bone set up on a Honda the other day.
Essentially I'm thinking, get the double wishbones, diff and uprights out of a wreck. Make a square frame around the diff' that gives the right track. Mount the wishbones on the vertical parts of the square. Keep in mind available drive shafts when makeing up the square(there must be many lengths to choose from). Hopefully You should get with-in a few mm of std track width with off the shelf (wreck) parts. The only fabricated part is the frame which even I could make up in a day with no special tools except a set square and a welder. Conceiviably not even a welder. Bolt together at corners, use slotted holes to aid alighnment. Same for the links from the bottom front corners to the spring hangers. When it's all alighned then maybe some welds, or just a second bolt hole drilled. The diff itself should stop any wracking.

The Miata has a bolt in rear subframe which houses its IRS completely. They also came with plenty of Torsen type limited slip diffs and the V8 copnversion Miatas used the pumpkin from a Ford T-bird supercoupe. Also, the Rx7 II rear unit drops into the Miata assembly with little to no trouble.
The Miata rear end has disc brakes.

The miata IRS measues about 5 inches wider hub face to hub face than does the MGB rear axle. If you were running some serious flares, this wouldn't be too much of an issue though.

The real challenge, in my mind, is finding a unit that will not require specially shortened halfshafts. If you can get the track right, then even altering or fabbing up a new cradle wouldn't be that big a deal.
Makes it a lot easier if the shafts wear or break.


Brian C
Brian Corrigan

Has anyone measured the Miata rear subframe for fit in the B? Would 200hp exceed the capability of the diff as is?
Michael Willis

I'd trade my Traction-Lok for a Torsen (in a second), but I think Miatas mostly come with 4.1 gears... (and Rx7's were also geared low.)

Some of the Mazda pickups had more appropriate (for us) ratios, if they're compatable...

Someone needs to work this all out - the Miata suspension would be a real improvement for MGB.

I remember a Vancouver shop doing "Monster Miatas" used Nissan 300ZX diff centres in the Miata cradle to give a higher ratio and strong enough for at least SBF power. I imagine some bracket changes would be necessary to support the diff unit but if a Miata cradle would fit an MGB body; you wouldn't have to worry about a suitable diff centre being available. Bob.
Bob Elwin

Good information on the Miata axle assembly here

Looks like it would adapt fairly easily to a B.
Bill Young

The stock torsen miata rear end (they also sold an open dif) that came with the 1.8 liter engines (90-93 were 1.6liters) will handle 220 turbo charged horsepower in miatas with nothing more than mechanical empathy, andn have been doing so for years with Flyin Miata Turbo Kits. For Higher power the RX7 Turbo II unit will fit. 3.6 gears were available in the early 80s 626 and you can still get them new from Mazdaspeed (if you race or autocross).

The miata V8 conversion folks also have a kit to swap in a Tbird Turbo coupe pumpkin, but that sort of defeats the purpose of doing this on the cheap. If the miata is too wide, then what is the point. For me, I would try it with flares.


Brian C.
Brian Corrigan


I believe many IRS are easily adaptable for a B. I have in hand a BMW strong IRS to cope with a lot of HP and can be easily tranform to coil-over. The main concern is track and bolt pattern that need some body modif. at fender level. Small SUV will have almost the pecfect track size but I doubt it can support a lot of HP.


Jean Guy Catford

Hawk cars in the U.K. offers an MGB IRS suspension upgrade, all sorted. Pricey, but all the engineering is built in.

They offer a lot of other interesting MGB upgrades, as well.
Tom Balutis

Funny thing about BMC: it thought IRS was a good idea for their 4x4s (Champ, Gispy and ANT) but not necessary for MGs.

Wonder if they ever considered the Gipsy's IRS when designing the B?
Tony Randall

In the fog of my mind I seem to recall that IRS was considered for the B and a prototype IRS car was built but...fiscal considerations.. brand in-fighting... never made produciton.

I'll try and locate where I read alll this..might be that big coffee table MG book.

With regard to the Miata IRS, i would prefer to modify the existing cage or build a new one so that all of the wearing bits are parts counter custom halfshafts etc..


Brian C.

Brian Corrigan

Your right Brian, I've read something similiar. Apparently those overly eleaboate big bump stop mounts were orrigionaly intended to locate rear coil springs on. It makes sence when you look at them with that in mind. The live rear axle was a cost compromise in keeping with Morris's approach of doing it cheaper than the competitor.
I've got to do something about locating that axle properly. I've become very cautious about accellerating out of corners. The outside spring is OK since it's not got muxh negative curve and is pretty much on the bump stop, but the inside spring can have quite a bit of positive curve to it and subsequently is prone to flexing under load ie serious axle steer. The back end can give quite a violent kick in the wrong direction at just the wrong time.

Peter, One of the books by David knowls details the development of the B and includes some oics of the prototype fitted with coil spring rear.
The rear styling was redone to eventually become the B we know today and this was brought about by the need to fit leaf springs solely for the reasons of cosr.

Cheers , Pete.
Peter Thomas

The rear of the car was lengthened by an inch or two also to fit the springs

I am interested in this subject also. I don't think any of the smaller SUVs with full time 4WD would be strong enough. The Toyota Soarer may warrant some investigation. Has anyone had a look at these?
Regarding the MGB factory prototype with IRS I seem to recall it was a modified version of the Triumph 2000 system.


This is a subject that has been around for a while in the race car arena, street roding and here.
First one must eximine why the IRS came about, what it has to offer and who and how is has been use and measure the success before jumping into a big project such as IRS.

The IRS came about to give a better ride than the solid axle. The advantage is that each tire operates independant from the other, so in rough roads it gives a better ride.
The SCCA gave the ok for Trans Am cars to use the IRS it was found by race teams using the IRS that it was lots of work to set up the IRS and the lap times did not improved, it was found that a well design Multi link rear suspension worked better. Trans Am cars do not use the IRS at present.
Anti Squat, anti dive and alignment are hard to come into a comprimise. Some designs of the IRS are not good
such as the 240,60,80Z they have exagerated squat characteristics and others have high roll centers and others have anti dive and wheel braking hop.
The IRS works well with very stiff springs which is what the R5 Corvettes use and the added links to the rear of the Vette improves it's handling and the wheels are conecdted via the leaf spring but still mantains the IRS portion thus improving the handling.
Overall the IRS is a very complicated system to just adapt from one to the other, many, many issues are involved in such swapt. It's better to specific design a IRS and test, test and test for the proper alignment and spring rate, the overall improvement would be a better ride in rough roads and more work to align properly for the chassis in which it has being install.

I am not trying to discourage anyone from doing it.
At was at Laguan Seca watching the Gran Am cars A German import with IRS was not faster than the new Mustang 4 link on the turns.
The solid axle is easy to adjust by changing spring rate and arm angle for a better spririted driving or normal club driving than the IRS toe, camber and link position.
Bill Guzman

Bill said: "The IRS came about to give a better ride than the solid axle." Would some subjective measure, such as "ride", be the reason every Formula 1 car in memory has used IRS? No, obviously not. Rough roads? Formula 1 cars don't drive on rough roads. IRS is required for competitive cornering.

IRS has been tried, with varying success, on sports cars for years and years and years. I believe the old Lotus Elan was fitted with IRS (via "Chapman struts", which are essentially "McPherson struts" fitted at the rear corners) for strictly PERFORMANCE reasons. Mr. Chapman had little or no interest in either "ride" or "rough roads".

Bill is absolutely correct that some IRS designs are "not good". That's understated. Swing axles (e.g. Triumph Spitfire, VW bug, Corvair) are dreadful. (High roll center plus "jacking".)

A successful IRS design for MGB would require either skillful engineering or as Bill suggests, a lot of testing. (It may require irrational amounts of money and taking a cutting torch to MGB's belly...) But the end result could be a lot more than just a better ride on rough roads. IRS offers lower unsprung weight plus camber characteristics that change favorably with both vertical suspension movement and body roll. In other words IRS offers the promise of faster cornering, and that's why I encourage someone to engineer and optimize IRS for MGB.

Good points Curtis. I am not trying to discourage anyone from building an IRS system for the MGB

The early days of Formula 1, 2, 3 they were using a de Dion type suspension, which is very complex, as the chassis evolve the cars went to an IRS. At present is a combination with a Rocker Arm type suspension and inboard brakes front and rear.

The Lotus used the IRS due space constraints and chassis design. The IRS in open wheel cars becomes part of the rigity of the chassis, same as some sport cars. The IRS in an open wheel car has very little motion. Observed the cars when they come into the pits and they are jack up, the wheel has 0 travel, when the car is lowered it has 0 compression. The suspension travel is measured in 1/8” to ¼” of an inch, not much of an IRS in those cars, which they compress when high loads of down force is applied to the chassis. The same principal is used in other sports cars with IRS systems. In sport car racing the travel is bit more.

Advantages of IRS
Less unsprung weight due to the differential is mounted solid and does not travel up and down. (when properly design)
It takes less room allowing being use a confine space.
Better ride on rough roads in the daily use.

It’s more complex to design, build and set up correctly.
The ability to deliver power to the ground. The basic geometry of an IRS is such that only small amounts of anti squat can be used, so high output cars without weight bias will have problems getting the power to the ground.
The latter applies to a road race car with IRS coming out of a turn, that’s why the IRS is set up almost solid in a sport car to improve traction and minimize squat during acceleration, not much of an IRS
The later 96 C4, C5 and the new C6 Corvettes have extra links to eliminate the anti-squat characteristics of the IRS. Same as Ferrari and other high end sport cars.

The IRS for the street is very different that IRS set up for a road course.
Simplicity some times is better, Fords new Mustang uses a 3 link which can be a bit tricky to set up correctly but simpler than an IRS by far. The masses can play with it, tune it for almost anything, Drag racing, road racing or just pleasure, dificult to do the same with a IRS.

Bill Guzman

I'm still looking at the small SUVs, saw a Hyundai today that looked interesting but again, a 2wd and no rear diff and a question occurred to me. If the 2wd vwesions of these vehicles is fwd then in the awd does the rear axle just go along for the ride, or do they balance the power to the rear? Could make a big difference in the strength of the rear diff. I'd guess that Hyundai went close to 4000lbs.

Jim Blackwood

It is worthwhile to keep in mind the cost factor as well which is THE prime concern for major car manufacturers.
IRS is easy to get wrong and to resolve such mistakes especially for a one off is no small undertaking even for suitably qualified and very experienced people.

Just a thought or two.

Cheers , Pete.
Peter Thomas

I've always figured the simplest method would be to take a well sorted out IRS with a track not too much wider than needed and move the pivot points inward enough to narrow it the necessary amount. That way none of the geometry changes and it should work as before. Of course this necessarily depends on the exact arrangement and it would work on some but not on others, but the second advantage of this method is that any tuning bits developed for the donor car would be usable to correct such things as accelleration squat, etc. I still plan to go ahead with my modified T-bird IRS but it doesn't hurt to look at others.

BTW, Dan can we crawl under your RAV4 for a look see?

Jim Blackwood

The trouble with MGB's being so low to the ground is that your head lights clearly show all the various IRS's that everyone else seems to have.
I think I have have IRS envy.

I'm still thinking about the interchangeability of front and back drive shafts on largish 4WDS. Shorter drive shafts at the front cause the front gearbox plus diff' is generally wider than the rear diff'.Maybe even use the front suspension and uprights. Would make for an interesting way of alighning the wheels. You could even set it up so that as the car rolled a little in the turns the rear wheels would steer.

"BTW, Dan can we crawl under your RAV4 for a look see?"

Yeah, but you won't see much. My RAV4 is only a 2WD/FWD. Not much need for 4WD here in Tennessee.
Dan Masters

Speaking of "IRS envy"... I was just doing some surfing and came across this photo.

Chapman strut:

"Spyder Cars" sells all sorts of nifty suspension bits for updating Lotus Elans, and their engineering seems pretty well sorted out. Track (width) should be similar, but Elans weigh a lot less than MGBs (about 1500# vs about 2400#), so different spring rates and shock valves would be required.

I'm sure these folks would be be happy to put together a kit for MGB... for a price.


I would suggest a strut brace to link the two towers and provide some extra rigidity.

Cheers , Pete.
Peter Thomas

I dunno Dan, might be more ther to look at than you think.

Jim Blackwood


You're not thinking of converting your MGB to front wheel drive are you?
Dan Masters

I always thought an All Wheel Drive B would be cool, but one would probably have to start ground up and make an actual frame for the car then fit B body panels overtop of it.

I've looked at some of the smaller SUV's and it would seem to me that the mgb frame rails would seriously get in the way of making things work properly. Also, few of those cars have 200hp motors with significant amounts of torque like we have. How long would they hold up to the kind of abuse we throw at cars?

I think that it is interesting that the new Mustang as well as next years Shelby Mustang (Cobra) does without an IRS.
Michael S. Domanowski

The mustang is on a version of the floorpan the Jaguar S-Type uses, which does have IRS, but on the grounds that

a)Mustang onwers prefer live axles


b)It's cheaper with a live axle

They've decided on the live axle. I guess it makes more sense for drag-racing and traffic light grand prix, which is where most Mustangs will be used in anger.


This thread was discussed between 16/04/2005 and 24/05/2005

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